Modeling and measurements of angular truncation for an aerosol albedometer

In this work, we examine the angular truncation behavior and present correction factors for the aerosol albedometer previously developed in our laboratory. This new instrument makes simultaneous measurement of extinction and scattering coefficients (b_ext and b_scat) on dispersed aerosol samples. The aerosol extinction coefficient is measured with cavity ring-down spectroscopy (CRDS), and the scattering coefficient is determined through the integrating sphere nephelometer. However, all nephelometers are not able to collect light scattered from an aerosol sample very near the forward (0 °) and reverse (180 °) directions, due to the geometrical constraints. This can result in systematic underestimation of scattering coefficient known as truncation error. In order to account for this problem and describe scattering by aerosols more precisely, correction factors (C) for this angular non-ideality have been theoretically developed. Truncation angles (θ) were calculated upon consideration of the geometry of the sphere nephelometer. As truncation error largely depends on particle size and refractive index, C values were computed for a series of spherical, homogeneous aerosol particles with different known particle sizes and refractive indices by Lorenz-Mie theory. Measurements on size-selected, laboratory generated aerosols of known size and composition allowed empirical measurement of truncation correction factors to compare with the Mie model results. Results indicate the model we built overestimates the fraction of light not collected by the sphere. Empirically observed correction factors of ≤ 1.12 for particles with size parameters (α) < 6 were determined. In addition, the effect of number of particles within the probe beam on the suitability of correction factors was also examined. Observations support the hypothesis that particles are rapidly transported / mixed through the probe beam, and measurement integration times as short as 52 s yield data that is indistinguishable from the probe region being homogeneously filled with aerosol, even at very low particle concentrations.